Screen printing is a well established method of transferring a fluidized or other media into a pattern on the surface of an object. Screen printing is applied in many forms of manufacturing, ranging from the printing of inks on cloth, such as T-shirts, to printing of solder paste and adhesives onto printed circuit boards (PCBs) for the assembly of electronic products. A wide variety of equipment and tooling may be used for a wide range of screen printing applications. As particularly described in this application for exemplary purposes only, screen printing is used in the manufacture of PCBs, which are sometimes also referred to as printed circuit assemblies (PCAs). As shown in FIGS. 1A and 1B and generally known in the art, the components 3 that are attached to and perform the operational functions of a PCA 1, are attached to a substrate 2. One of the most common ways to attach the components 3 is to place a series of solder paste deposits 7 directly on the substrate 2 or on a pad 5 attached to the substrate 2. The pads 5 may connect to leads 8 on the substrate 2. After the components 3 or component leads 6 are placed into the paste deposits 7, the assembly 1 may be heated above the liquidus temperature of the solder, causing the solder paste deposits 7 to flow and join the components 3 to the pads 5 on the substrate 2.
As is further known in the art and generally illustrated in FIG. 1, the components 3 to be attached to the substrate 2 are very small and getting smaller. Generally, the steps used in the process are known in the art, as described below.
A stencil (also referred to as a screen) is created with an aperture or a plurality of apertures defining a pattern that is to be `printed` onto a surface. The stencil is placed onto the surface upon which a material is to be deposited in a pattern. The material may be in a liquid or solid or solid/liquid composition. In the case of the preferred embodiment, the material that is to be deposited comprises fine particles of solder powder mixed into flux.
The stencil is generally placed substantially parallel to the surface and may contact the surface. The aperture or apertures are aligned on the surface to create the desired pattern. For gravity-driven printing, the stencil is generally placed on top of the surface. The material to be deposited is then placed on top of the stencil for deposition into the aperture or apertures. Various methods may be used to move the material from the top of the stencil and place the material into the aperture or apertures, as are generally known in the art. For example, squeegees are often used in various ways to move material into the aperture or apertures. Once the apertures are filled with material, excess material may be removed from the top of the stencil so that substantially all of the material that remains proximate the stencil is in the aperture or apertures.
Lastly, the stencil is removed from its proximity to the surface of the substrate, leaving the material deposited on the surface of the substrate.
Various methods and equipment have been invented to automate the process described above, with many different approaches, as are known in the art. Many improvements in the art have resulted in an increase in the efficiency of the process. For example, machines have been invented to hold the stencil, align the stencil to the surface of the substrate, more accurately place and remove the material into the apertures, and to separate the stencil from the surface of the substrate. However, none of these has solved the problems that the present invention solves.
The screen printing process is made more difficult as the size of the apertures decreases and the size of the surface area on the objects decreases. This is particularly true in manufacturing electrical and electronic assemblies. One particular problem that the present invention solves is that the material that is to be deposited onto the surface tends to stick to the sides of the apertures in the stencil. This problem has two particular outcomes that the present invention solves. First, the deposited material may slump or otherwise move outside the area defined by the aperture after the stencil is removed. For electronic assemblies, this can have disastrous consequences and require rework of defects. Secondly, the shape of the remaining material may cause problems. Preferably, the material that is left on the surface will have a uniform surface defined by the stencil apertures, and the remaining top surface of the material will be substantially flat. For PCBs, the material that is deposited on the surface is generally referred to as a solder paste deposit. Uniformity is very important for deposits so as not to create an area of conductivity where that is not desired. Further, it is important that the deposit have a uniform top surface to enhance the attachment of electronic components.
FIGS. 2A, 2B, 2C, 2D and 3 are simple illustrations that show some of the basic steps in the process. FIG. 2A shows a three-dimensional view of a stencil 10 that has multiple apertures 11. FIG. 2B shows a single aperture 11 in the stencil 10 from a cut-away view, slightly above an object 12. FIG. 2C shows the stencil 10 of FIG. 2B after it has been placed on the object 12 and material 13 has been placed in the aperture 11. Finally, FIG. 2D shows the stencil 10 that has been lifted off the object 12, leaving behind the material 13. The vertical length of the sides 15 of the aperture 11 in each of FIGS. 2B, 2C and 2D are relatively small compared to the target area 14 of the object 12 defined by the aperture 11. From a 2-dimensional view, the relative size of the target area is shown by the measurement A. When the stencil 10 is removed after material 13 is placed in the aperture 11, gravity and surface effects cause some of the material 13 to stick to the target area 14. To a lesser extent, surface effects cause the material 13 to stick to the aperture sides 15. If the target area 14 is much larger than the sides 15 of the aperture 11, the effect of material 13 sticking to the sides 15 is of less practical concern.
However, FIG. 3 illustrates the problem presented when the sides 25 of the stencil 20 aperture 21 become relatively larger when compared to the target area 24 of the surface 26 of the object 22. (as above, the target area 24 is proportional to measurement B.) This potential problem is generally due to shrinkage in the size of the components to be mounted. Here, the surface effects of the material 23 contacting the sides 25 are relatively larger than the surface effects of the material 23 contacting the surface 26, resulting in a tendency for the material 23 to stick to the sides 25 of the aperture 21, causing a number of problems or potential problems. As mentioned earlier, the material 23 may slump and migrate outside the area defined by the aperture 21 causing conductivity problems. Further, the surface area of the resulting deposit 23 may not be uniform, potentially creating problems in attaching components.
The limitations of this process continue to be challenged as the apertures and the resulting deposit area decrease in size. There are factors other than geometry that may impact the release of the material. Examples include the shear to tact ratio of the material, the surface finish of the stencil, and the cross sectional geometry of the stencil aperture sides.
To date, attempts to solve the problem have focused on changing the stencil release speed, changing the surface finish of the stencil, and changing the cross-sectional geometry of the aperture using aperture sides that are non-vertical.
A first known approach is the use of a slow separation between the stencil and the surface. The slow separation utilizes gravity to assist in the release of the material by allowing the weight of the material to overcome the shear force at the interface between the material and the side wall of the stencil aperture. The major detriment of this approach is that it inherently makes the process take longer.
A second approach known to assist with release of the printable material is to modify the surface of the stencil and at least change the surface finish of the surface of the aperture sides. Two examples of this approach applied to metal stencils would be electropolishing and nickel plating the surface after creating the apertures. This approach helps the release, but may be expensive and ineffective.
A third known approach to assist with release of the printable material is to design the cross section of the aperture in a trapezoidal shape, where the area defined by the aperture at the surface of the object side is larger than the area defined by the aperture at the stencil top side.
Thus, what is desirable, is a means to increase the speed of release of the material without altering the geometry of the deposition, and increase the quality of the resulting material deposition.